Orthogonal frequency-division multiplexing OFDM employs discrete multi-tone modulation. With OFDM, the tones are modulated on a large number of evenly spaced subcarriers using some m-ary of quadrature amplitude modulation (QAM) or phase shift keying, for example. OFDM allows only one user (transceiver station) on a channel at any given time to accommodate multiple users, an OFDM system must use time division multiple access (TDMA) or frequency division multiple access (FDMA).
Orthogonal frequency-division multiplexing access (OFDMA) is a multi-user version of OFDM that allows multiple users to concurrently access the same channel, where a channel includes a group of evenly spaced subcarriers. OFDMA distributes subcarriers among users (transceivers) so multiple users can transmit and receive within the same single RF channel (TDD) or different RF channel (FDD) on multiple subchannels. The subchannels are further partitioned into groups of narrowband “tones.” Typically, the number of tone in a subchannel is dependent on the total bandwidth of the subchannel.
A conventional star network includes of one master node and multiple slave nodes. The master communicates with all of the slaves node, and the slave nodes only communicate with the master nodes. Such networks can use orthogonal frequency division multiple access (OFDMA), and time division multiple access (TDMA) symbols.
In applications that require high reliability with relatively low latency such as factory automation, data from all the slaves need to be retrieved within a given latency constraint (delay). The latency constraint is equal to a communication cycle.
Polling of slaves one-by-one may not satisfy the latency requirement of the applications.
Also, in case when transmission from a slave node fails, that particular slave should be given a retransmission opportunity to meat the required transmission reliability without violating the latency constraint.